1. Field of the Invention
This invention relates to cascaded pressure gradients and pressure staging in hydrogen absorption systems for improved energy utilzation resulting from the heats of absorption for heat effects, and from the heats of desorption for refrigeration.
2. Brief Description of the Prior Art
U.S. Pat. No. 3,943,719 describes hydride-dehydride-hydrogen (HDH) cycles used for the production of simultaneous and continuous power and refrigeration by means of thermochemical compression utilizing hydridable materials. For continuously supplying relatively high pressure hydrogen gas, a plurality of hydride-dehydride reactors are provided and are operated in out-of-phase or staggered sequence so that during the period when low-pressure, relatively cool hydrogen gas is being charged to one of the reactors, another is being activated and another is being dehydrided to produce high pressure hydrogen gas. The pressure energy of the gas thus developed in the hydride reactors is used for continuously developing power and refrigeration, following which the hydrogen gas, at reduced energy, is recycled to the reactors to recommence the HDH cycle. In order to chemically compress the hydrogen gas in the form of its hydride, a low-grade thermal source is utilized to supply heat to the several reactors.
In work carried on by Brookhaven National Laboratory for the United States Government, a high efficiency power conversion cycle, using hydrogen compressed by absorption on metal hydrides in a regenerative closed hydrogen Brayton cycle, has been proposed. In the cycle, hydrogen is thermochemically compressed using a low-temperature thermal energy source, such as geothermal or solar energy, is next regeneratively heated, and is then further heated by a high-temperature thermal source, such as fossil or nuclear energy, and then is expanded, reheated, and expanded again. The hydrogen is returned through the regenerators and then recompressed in the hydrides. Overall efficiency approaches 30 percent. However, high temperature energy efficiency, defined as the work output divided by the high temperature thermal input, approaches 90 percent.
In U.S. Application Ser. No. 715,231, filed on Aug. 18, 1976, now U.S. Pat. No. 4,055,962 hydrogen-hydride absorption systems are described which are comprised of two hydride components. Methods for deriving refrigeration and heat pump effects are described. One hydride component system operates as the equivalent of a mechanical refrigeration system. A low temperature thermal sink is provided by supplying the heat of desorption to a reactor of this hydride component system. The hydride component system then rejects energy as the heat of absorption at an intermediate temperature. The second hydride component system operates as the equivalent of a heat engine cycle. A thermal source at a relatively high temperature supplies the heat of desorption to this component system and heat is rejected as the heat of absorption at an intermediate temperature.
The Carnot cycle defines the limit of thermal efficiency not only for heat engine cycles and mechanical refrigeration cycles, but also for absorption cycles. The maximum efficiency for any cycle generating work from any thermal energy input is limited by the Carnot efficiency, which is defined as the net work produced, W.sub.net, divided by the heat input, Q.sub.H, and is equal to (Q.sub.H -Q.sub.Amb)/Q.sub.H =W.sub.net /Q.sub.H =(T.sub.H -T.sub.Amb)/T.sub.H, where Q.sub.Amb is the available ambient heat sink and T.sub.Amb is the ambient temperature. For mechanical refrigeration, the Carnot limit of thermal efficiency is defined as the heat absorbed by the cooling load, Q.sub.L, divided by the net work input, -W.sub.net, and is equal to Q.sub.L /(Q.sub.Amb -Q.sub.L)=Q.sub.L /(-W.sub.net)=T.sub.L /(T.sub.Amb -T.sub.L). The maximum efficiency of an absorption cycle is thus defined, with the work output of the expansion device in the heat engine system equal to the work input of the compressor of the mechanical refrigeration system, and is therefore Q.sub.L /Q.sub.H =(T.sub.L /T.sub.H)(T.sub.H -T.sub.Amb)/(T.sub.Amb -T.sub.L). The higher the efficiency in the heat engine cycle, the lower the refrigeration load may be without significant energy costs. The high efficiency of the heat engine cycle requires a large temperature differential between a thermal source and a thermal sink.
In the method of operation of the heat pump absorption cycle, the heat engine cycle equivalent of one hydride component system operates from an intermediate temperature thermal source which provides the heat of desorption. The heat of absorption is rejected to a low temperature thermal source. The second component hydride system which operates as an equivalent mechanical refrigeration system has, as its refrigeration load, a thermal source at an intermediate temperature which provides the heat of desorption. The heat of absorption is rejected at a high temperature which is the heat pump effect.
A heat pump system is essentially a mechanical refrigeration system with a different objective in view. The rejected energy in the refrigeration cycle becomes useful energy. The heat input is to the evaporator from some ambient heat source. The efficiency is defined as the useful heat rejected, Q.sub.H, divided by the net work input, -W.sub.net, which is equal to Q.sub.H /-W.sub.net =Q.sub.H /(Q.sub.H -Q.sub.Amb)=T.sub.H /(T.sub.H -T.sub.Amb). If an absorption system is again considered as a combination heat pump and heat engine system, with the heat engine operating with a heat source at ambient conditions and a heat sink at some lower temperature T.sub.L, the efficiency of the heat engine would be Q'.sub.Amb /W.sub.net =Q'.sub.Amb /(Q'.sub.Amb -Q.sub.L)=T.sub.Amb /(T.sub.Amb -T.sub.L). The combined absorption system efficiency can be defined again with the work output of the heat engine system equal to the work input to the heat pump system as Q.sub.H /Q'.sub.Amb =(T.sub.H /T.sub.Amb)(T.sub.Amb - T.sub.L)/(T.sub.H -T.sub.Amb), where the Q'.sub.Amb is only the heat input to the heat engine system.
In work carried out at the U.S. Government Naval Underwater Systems Center, a one component hydride system operating in a conventional heat pump cycle with mechanical compression of the hydrogen has been described in a paper of 1975. Dieter Gruen, et al., of Argonne National Laboratory, have described in a paper presented in April of 1975, a heat engine cycle based on hydrides which closely follows the concept in U.S. Pat. No. 3,943,719. In March of 1976, a two component hydride system operating non-continuously to produce heat pump effects at night from a solar system is described in Gruen. A refrigeration system is also alluded to briefly and broadly. In September of 1976, a third paper by Gruen et al. describes a continuous two component hydride system for producing refrigeration and heat pump effects. This work relates closely to the disclosure of U.S. patent application Ser. No. 715,231 filed on Aug. 18, 1976.
In U.S. patent application Ser. No. 715,231, the methods described are the most direct means of achieving refrigeration and heat effects when a temperature differential between a thermal sink and source is available. The higher the temperature of the heat effect, and the lower the temperature of the refrigeration sink, the larger the temperature differential of the thermal sink and source must be. The result is to have a high efficiency in the heat engine cycle by way of the large temperature differential of the thermal source and sink, thus allowing an efficient heat pump cycle to give a high temperature thermal source, or refrigeration at a low temperature sink.
With a small temperature differential between the thermal source and sink, the heat engine cycle is much less efficient but the heat effect or refrigeration can still be achieved at the expense of having larger heat inputs from the thermal source and larger heat outputs to the thermal sink. However, these larger thermal inputs and outputs need not be to disadvantage when such supplies are much more readily available than high temperature differences between the thermal sources and sink.
In the system disclosed in U.S. patent application Ser. No. 715,231, the energy utilization factor (E.U.F.) for the direct refrigeration cycle without pressure staging, is possibly as high as 3.0 if rejected thermal energy is used for such things as home heating. The useful energy would consist of 2 units of heat at an intermediate temperature and a unit of refrigeration at a low temperature, with only one unit of heat input at a high temperature. A pressure staged system would allow a higher E.U.F. of possibly 5.0 or higher. With a high temperature heat input of 1 unit at 290.degree. F., three units of heat at 120.degree. F. could be rejected and 2 units of refrigeration at 32.degree. F. would be possible.
The combination of a pressure staging heat pump hydride cycle combined with a heat engine cycle would allow the heat engine cycle to operate at, for instance, a thermal input at 220.degree. F. and heat rejection at 80.degree. F. The pressure staging heat pump cycle, using the rejected heat as a heat input, would reject heat at 120.degree. F. The high temperature heat input, being 1.40 units, would produce 1.20 units at 120.degree. F. and 0.20 units of electrical output. This would be the maximum possible according to the first law of thermodynamics. If the heat engine cycle were to be made to operate between 220.degree. F. and 80.degree. F., instead of 220.degree. F. and 120.degree. F., the Carnot efficiency would be 15 percent instead of the 20 percent achieved--a 25 percent drop in the efficiency.